The invention relates to epoxy resin mixtures for producing composites by the injection process.
Composites based on epoxy resins and on inorganic or organic reinforcing materials have achieved great importance in many areas of engineering and of daily life. The reasons for this are on the one hand the relatively simple and reliable processing of the epoxy resins and on the other hand the high level of mechanical and chemical properties of the cured epoxy resin materials, which permits adaptation to different applications and advantageous utilization of the properties of all of the materials involved in the composite.
Especially in the production of small numbers of large-surface-area parts, as used for lightweight construction of aircraft, ships and vehicles, and also in the form of materials used for housings, insulation or construction in the electrical and building industries, care has to be taken that shaping processes are simple and cost-effective. Injection or RTM technology has proven successful here (RTM=resin transfer molding), permitting economic operation and giving smooth internal and external surfaces. In this process, epoxy resin mixtures are injected into a mold previously provided, if necessary, with the required inorganic and/or organic reinforcing materials. Reinforcing materials which can be used here are glass fibers, carbon fibers, aramid fibers and/or other reinforcing fibers, the selection of the fiber material depending on the mechanical requirements placed upon the products. Wood and natural fibers can also be suitable. For lightweight construction in particular, use may also be made of foams, such as polyurethane foams or PVC foams.
Injection technology is particularly advantageous when the epoxy resins used can be processed at room temperature and cured at low temperatures without applying pressure. In this case cost-effective polymer molds can be used. However, the resin formulations required have to have very low viscosity and have good flow performance and wetting performance with respect to the reinforcing materials, and develop a good bond to these. In addition, they have to cure without application of pressure and, after release from the mold and, if desired, postcuring, must have excellent mechanical properties.
Another requirement which has achieved increasing importance in recent times is that for low combustibility. In many areas this requirement is given first priority because of the danger posed to life and property, for example in materials used for the construction of aircraft, ships, motor vehicles or rail vehicles, in particular if the vehicles are used as a means of public transport.
The cured resins have to pass various tests to assess the combustion performance of the materials. For electronic products, for example, the UL 94 V combustion test is required, predominantly with V-0 classification. For polymers in rail vehicle construction, the combustion test of DIN 5510 has to be carried out. Here, for example, S4 classification requires that, after flame application for 3 minutes, the material becomes extinguished within a few seconds, the diameter of the flame-damaged area is less than 20 cm, the material does not form drops and the integral smoke gas density causes light scattering of less than 50%. For construction materials, the specifications in DIN 4102 apply.
However, these requirements are difficult to fulfill. All of the known flame-retardant cured epoxy resin materials used in industry therefore comprise up to 20% of bromine in the form of brominated resin components. Considerable amounts of antimony trioxide are often also used as a flame retardant with synergistic effect. The difficulty with these compounds is that although on the one hand they have excellent effectiveness as flame retardants, on the other hand they also have very hazardous properties. For example, antimony trioxide is on the list of chemicals which cause cancer. When aromatic bromine compounds decompose thermally they produce not only bromine radicals and hydrogen bromide, which cause severe corrosion, but when decomposing in the presence of oxygen in particular the highly brominated aromatics can, even more significantly, also form highly toxic polybromodibenzofurans and polybromodibenzodioxins. Considerable problems are also raised by the disposal of waste and used materials containing bromine.
For these reasons there has been no lack of attempts to replace the bromine-containing flame retardants by less problematic substances. Examples of those which have been proposed are fillers with extinguishing gas action, such as alumina hydrates (see: J. Fire and Flammability, Vol. 3 (1972), pages 51 ff.), basic aluminum carbonates (see: Plast. Engng., Vol. 32 (1976), pages 41 ff.) and magnesium hydroxides (EP-A 0 243 201), and also glass-forming fillers, such as borates (see: Modern Plastics, Vol. 47 (1970), No. 6, pages 140 ff.) and phosphates (U.S. Pat. Nos. 2,766,139 and 3,398,019). However, the disadvantage attaching to all of these fillers is that they considerably impair some of the mechanical and chemical properties of the composites. Organic phosphorus compounds, such as phosphoric esters, phosphonic esters and phosphines, have also been proposed as flame-retardant additives (see: W. C. Kuryla and A. J. Papa, Flame Retardancy of Polymeric Materials, Vol. 1, pages 24 to 38 and 52 to 61, Marcel Dekker Inc., New York, 1973).
Epoxy resins may also be made flame-retardant by using reactive organic phosphorus compounds, such as epoxide-group-containing phosphorus compounds, which can be anchored in the epoxy resin network. European Patent 0 384 940 discloses epoxy resin mixtures for use in printed circuit board materials which comprise a phosphorus-free polyepoxy resin in combination with an epoxide-group-containing phosphorus compound of the structure ##STR1##
and with a specific aromatic polyamine (in the form of an isocyanuric acid derivative) as hardener. DE-A 43 08 184 and DE-A 43 08 187 and, respectively, the corresponding publications WO 94/21706 and WO 94/21703 disclose epoxy resin mixtures which comprise a phosphorus-modified epoxy resin (epoxide value: from 0.02 to 1 mol/100 g) in combination with the polyamine mentioned. The phosphorus-modified epoxy resins here have been built up from structural units which derive on the one hand from polyepoxy compounds (having at least two epoxide groups per molecule) and on the other hand from phosphinic, phosphonic and pyrophosphonic acids or from phosphonic monoesters and, respectively, from phosphinic anhydrides and phosphonic anhydrides. Other epoxy resin mixtures which comprise phosphorus-modified epoxy resins and aromatic amines as hardener are known from the publications WO 96/07684, WO 96/07685 and WO 96/07686. All of these epoxy resin mixtures are preferably processed from a solution; in bulk they are solid or highly viscous.
There has also been no lack of attempts to develop casting resins based on the phosphorus components mentioned. For example, there are known epoxy casting resins which can be cured by anhydride and comprise phosphonic anhydride as hardener or are obtained by modifying epoxy resin components or hardener components with phosphorus compounds (see in this connection: DE-C 42 37 132, DE-A 195 06 010, WO 96/07678 and WO 96/23018). These casting resins are predominantly highly viscous and, without solvent, cannot be processed below temperatures of&gt;60.degree. C.; temperatures of&gt;80.degree. C. are required for their curing. Low-viscosity epoxy resins which cure using amines at room temperature and are flame-retardant after curing are not yet known.